Ball joints employing ball studs and sockets are commonly used in mechanical linkages where a first member pivots relative to a second member, and are therefore widely used in motor vehicle suspension linkages. A ball stud is fixed relative to one member and a socket fixed relative to the second member, with the socket pivoting on the ball stud.
Ball joint assemblies employed in motor vehicle suspension linkages sustain very high loads and therefore typically have a ball joint socket formed substantially of steel. Steel sockets may have plastic bearing liners. The steel socket is crimped around the ball end to retain it, and the bearing liner is formed by injecting plastic between the socket and ball stud. One ball joint assembly is formed principally of plastic, with the socket being reinforced by a cup shaped retaining member through which a shank of the ball stud passes. An advantage of a ball joint socket formed mostly of plastic is that it is highly resistant to corrosion and maintains a like-new appearance, even after many years of use.
Because the bearing liner is molded around the socket and ball end, the thickness of the liner at any one location depends on the relative position of the ball end to the socket during molding. Slight variations in thickness of the liner have a very significant effect on ball joint life. If the liner wears through to produce metal to metal contact, the ball stud may be damaged. The bearing liner thickness, and therefore the durability of the ball joint assembly, are consequently highly dependent upon the relative positioning of the ball end and socket.
It is desired to provide a ball joint with a socket formed largely of plastic sufficiently strong to be employed in motor vehicle suspension linkage applications and eliminating the variability concerns associated with the use of a metal retaining member.